Saponification
Saponification is a chemical process in which soap is produced from fats through a reaction with alkaline substances. The term originates from the Latin word "sapo," meaning soap. During this process, fats, which are esters of long-chain carboxylic acids and glycerol, undergo hydrolysis, typically using a base like sodium hydroxide (NaOH). The reaction involves the nucleophilic attack of hydroxide ions on the ester, leading to the formation of soap and alcohol. Different types of fats can yield varying soap products, with characteristics influenced by the nature of the fat and the base used. Saponification is not only a crucial chemical reaction but also a historical practice, once commonly performed in households before the widespread availability of commercially produced soaps. Understanding saponification is essential for recognizing how soaps work, especially in their ability to emulsify oils and grease, which is key to their cleaning properties.
On this Page
Subject Terms
Saponification
Saponification is a chemical reaction during which soap is formed from fats. The term saponification comes from the Latin sapo, meaning “soap.” The soap produced during saponification is an organic salt. Saponification is an important type of chemical reaction that can occur between different types of fats and alkaline, or base, materials. This reaction helps people produce soap that is used for personal care and household products. The chemical reaction that takes place is a form of hydrolysis, which happens when water breaks chemical bonds.
Background
Saponification often occurs with an ester or a fatty acid. An ester is an organic compound that reacts with water to create acids and alcohols. Esters are usually created from carboxylic acid, and they form when an –OH group is replaced by an –O–alkyl group. When carboxylic acid and alcohol react, they can form esters. Esters come in many different forms, including colorless liquids, waxes, and fats. Fats and oils are actually esters of long-chain carboxylic acids and glycerol. The esters used in saponification are these fats and oils. Animal tallow (or fat), olive oil, and coconut oil are all esters that people commonly use in the saponification process.
Overview
Saponification is the hydrolysis of an ester. Hydrolysis is a type of chemical reaction in which a molecule of water (or a weak acid or base) breaks one or more chemical bonds. In saponification, the esters are broken down by hydrolysis. Ester hydrolysis can occur in the presence of either an acid or a base. However, saponification is a base-promoted ester hydrolysis, which means a base is required to complete saponification.
Saponification can occur between different esters and bases. One common ester used in saponification is a triglyceride, which is an ester formed from glycerol and three fatty acids. The chemical compound OH (hydroxide) is a base and can be used in saponification. The base NaOH (sodium hydroxide) is also commonly used. In this example of saponification, the OH (in either hydroxide or sodium hydroxide) will attack the ester. The ester has carbon in it, and the OH nucleophilically attacks this carbon. A nucleophilic attack is when an electron-rich nucleophile bonds with an atom or a group of atoms. In saponification, the base is the electron-rich nucleophile, and the ester is the electron-poor acceptor.
The OH attaches itself to the ester. When it does, it shifts some of the electrons around in the ester molecule. When the electrons shift, part of the ester molecule leaves. So, the OH replaces part of the ester molecule. This break of the bond is the hydrolysis part of the reaction. Then, deprotonation, or the transfer of protons, happens in the reaction. The products of the reaction are soap and an alcohol, which, depending on the reactants, could be glycerol.
People often use heat in the saponification process to help the chemical reaction occur reliably. Saponification is the same type of chemical reaction no matter the type of base or fat is used to create the reaction. However, the soap and alcohol produced by the chemical reaction is different based on the fats and bases used in the reaction. Potassium soap is softer compared to sodium soap (so soap formed with base KOH is softer than soap created with the base NaOH). Also, different types of fats create different types of soaps.
People commonly call many household cleaners “soap” today, but many of those cleaners are actually detergents and not soaps. However, both soap and detergent have similar chemical makeups, allowing them both to remove oil and dirt. The dirt that sticks to skin and clothing often sticks because it has a layer of fat, or lipids, keeping it stuck to a surface. Soaps and detergents both dissolve nonpolar substances such as fats, oils, and greases. The molecules of soaps and detergents contain a non-polar (hydrophobic, meaning “water hating”) hydrocarbon ends, and polar (hydrophilic, meaning “water loving”) ends. The non-polar ends of the molecules surround molecules of lipids. Both the non-polar ends of the soap molecules and the lipids are hydrophobic so they will not dissolve in water by themselves. Yet, because the non-polar ends and the lipids are similar, the non-polar ends begin to dissolve the lipids. The polar (or “water loving”) ends of soap (or detergent) molecules do dissolve in water, and they help to emulsify the lipids that have been partially dissolved by the non-polar ends of the molecules.
The soap that results from the saponification process works differently under different chemical conditions. Water has different dissolved materials inside it in different places. Some water, which people call hard water, has salts of calcium and magnesium dissolved inside it. These dissolved salts react with the salts in the soap to create a precipitate. A precipitate is a solid material created by a solution. The precipitate created by the reaction between the soap and hard water causes marks on bathtubs and other surfaces.
Saponification is a type of chemical reaction, but it is also an important process that humans use to create helpful products. Today, many people purchase soap from stores, but people used to create their soaps at home using saponification, though many did not understand the science behind the chemical reaction. In ancient Rome, people created soap mixing by rain water, potash, and animal tallow (or fat). Even at the turn of the twentieth century, many Americans still made their own soap using beef tallow and lye. Beef tallow is a triglyceride, which makes it an ideal fat for making soap. Lye is the name of a caustic base, usually sodium hydroxide (NaOH). People soaked wood ashes to make lye. Lye is highly soluble in water, making it a good base to use in the saponification reaction. Saponification of tallow and other hard fats produce harder, harsher soaps, but saponification of olive oil and other softer fats produces gentler soaps. For this reason, people who use saponification to create soap today often use olive oil and other liquid fats.
Bibliography
Clark, Jim. “The Hydrolysis of Esters.” Department of Education Open Textbook Pilot Project, 5 Jun. 2019, chem.libretexts.org/Bookshelves/Organic‗Chemistry/Supplemental‗Modules‗(Organic‗Chemistry)/Esters/Reactivity‗of‗Esters/The‗Hydrolysis‗of‗Esters. Accessed 27 Nov. 2024.
Domingues, Gabriel and Matt Schul. “Soap Lab.” ADM Biorenewables Education Laboratory, June 2014, www.engineering.iastate.edu/brl/files/2011/10/brl‗soaplab.pdf. Accessed 27 Nov. 2024.
Farmer, Steven. “Saponification.” Department of Education Open Textbook Pilot Project, 23 Sep. 2019, chem.libretexts.org/Bookshelves/Organic‗Chemistry/Supplemental‗Modules‗(Organic‗Chemistry)/Esters/Reactivity‗of‗Esters/Saponification. Accessed 27 Nov. 2024.
Gichuki, William Mwangi. "Saponification Reaction: Preparation of Soaps." Research Gate, Sept. 2022, dx.doi.org/10.13140/RG.2.2.10865.35684. Accessed 27 Nov. 2024.
Gunawardena, Gamini. “Saponification.” Department of Education Open Textbook Pilot Project, 5 June 2019, chem.libretexts.org/Bookshelves/Ancillary‗Materials/Reference/Organic‗Chemistry‗Glossary/Saponification. Accessed 27 Nov. 2024.
Hunt, Ian. “Hydrolysis of Esters.” Department of Chemistry, University of Calgary, www.chem.ucalgary.ca/courses/351/Carey5th/Ch20/ch20-3-3-1.html. Accessed 27 Nov. 2024.
“Saponification.” Illustrated Glossary of Organic Chemistry, www.chem.ucla.edu/~harding/IGOC/S/saponification.html. Accessed 27 Nov. 2024.
“What Is Saponification?” NLGI, 17 Jan. 2017, www.nlgi.org/faqs/what-is-saponification/. Accessed 27 Nov. 2024.